Many TPE compositions have been developed to combine the properties of soft rubbery materials with the properties of thermoplastics. Common TPE polymer combinations include polyvinyl chloride (PVC) and acrylonitrile butadiene (NBR) rubber; polystyrene/elastomer block copolymers; and polypropylene/ethylenepropylene diene terpolymers (EPDM) for example. A background article reviewing thermoplastic elastomers is found in Rubber Technology, 3rd Ed., by Maurice Morton, Van Nostrand Reinhold Co., NY (1987), p. 466.
There are references disclosing thermoplastic polyvinyl chloride (PVC) which has been modified to yield TPE properties. For example, high molecular weight plasticizers, or compatible rubbers such as NBR may be blended with PVC. Physical blending of polymers of acrylic acid esters with PVC to produce flexible polyvinyl chloride is known. Blending of an acrylate latex into a PVC polymer suspension and co-coagulating this mixture is also known.
A quite different method for combining an acrylate polymer with PVC involves polymerizing vinyl chloride in the presence of a preformed acrylate polymer. Intimate mixing of the PVC and acrylate phases including grafting between the phases is possible by the method. In some instances it is desirable to use a crosslinked acrylate rubber phase as this presumably preserves a more discreet soft phase dispersed within the vinyl "hard" phase.
This method of modifying PVC with acrylate ester polymers is described in U.S. Pat. No. 3,760,035. Blends of rubbery alkyl acrylate polymers and hard vinyl chloride resins are described by an overpolymerization method using a dispersion or suspension process without coagulation of the latex particles. A multi-stage process is claimed. In the first stage, a highly gelled or insoluble polyacrylate rubber is overpolymerized on polyvinyl chloride seed latex resulting in the deposition of a thin layer of rubbery acrylate polymer on the PVC seed particles. In the second stage the rubber-coated latex particles are subjected to an overpolymerization process. Preferably, the second stage product contains between about 35 and abouat 300% by weight of the hard resin deposit, or, otherwise stated, between about 3 to about 65% by weight, more often, from about 10 to about 25% by weight of the rubbery polyacrylate ingredient, and from about 97 to about 35% by weight, more often from about 90 to about 75% by weight of the hard vinyl chloride matrix resin. The product of the first stage acts as an impact modifier. This work is directed toward minimizing the effective level of impact modifier so as to preserve the desirable physical properties and chemical resistance of the PVC resin. Best results reportedly are obtained when the resin in final form contains about 5 to 6% by weight of crosslinked acrylate rubber relative to polymerized vinyl chloride. However, the claimed maximum for the acrylate component is 65%.
Surprisingly, the same invention directed toward soft, acrylate modified PVC materials made using the overpolymerization method is disclosed in U.S. Pat. No. 4,752,639. Flexible to soft articles are derived from thermoplastic compounds containing vinyl chloride-acrylic ester graft copolymers. Claimed compositions range from 20 to 60% by weight of acrylic ester, 40 to 80% by weight of vinyl chloride, and 0 to 30% by weight of other ethylenically unsaturated comonomers. Articles may optionally contain other plastics, fillers, processing aids and conventional additives. Solution, suspension or, preferably, emulsion polymerization processes are disclosed. The preferred forms are considered flexible, tough and "rubbery" but are not directed toward compositions which possess predominantly crosslinked elastomeric properties such as creep resistance, low temperature modulus and high temperature compression set.
No disclosures have been found which are directed toward compositions of PVC and acrylate ester rubbers in which the rubber phase dominates the thermo-plastic phase over a wide temperature range. Moreover, high molecular weight, crosslinked acrylate rubbers which can be melt processed as thermoplastic materials are heretofore undisclosed. The present invention involves the surprising discovery that it is possible to incorporate PVC onto particles of crosslinked acrylate rubber within the claimed range to give a material exhibiting the properties of a crosslinked elastomer over a wide temperature range, while exhibiting flow at elevated temperatures enabling the material to be melt-processed as a thermoplastic.